Mechanical ventilation of the lungs is a routine life-sustaining therapy in medical intensive care units for patients suffering from respiratory failure. Most respirators in use today for this purpose are of the positive pressure type in which the lungs are inflated by a positive pressure supplied by the respirator during insufflation, followed by passive exhalation as the respirator pressure is removed, whereupon the lungs and chest wall recoil from their inflated positions. In patients who retain some spontaneous breathing activity, lung inflation may also be due in part to the patient's own respiratory efforts. In this instance, the respirator acts as a mechanical assist to the patient, partially reducing the work involved and energy expended while the patient performs the breathing.
In order to maximize the mechanical efficiency of assisted respiration and minimize the patient's risk of exposure to excessive respirator pressure, it is important to synchronize the assisting pressure of the respirator to the patient's breathing efforts. Most modern respirators are equipped with triggering systems that detect the patient's attempt to inhale, and, in response, initiate the insufflation phase. A common design detects any precipitous fall in patient airway pressure at the end of exhalation as a sign of a spontaneous inspiratory effort. As the pressure falls below a threshold level, the respirator is triggered to begin an insufflation phase. To guard against mal-triggering, the pressure threshold cannot be made too sensitive.
For the airway pressure to fall below threshold, the patient must evacuate, by his own active inspiratory efforts, a sizable volume from the respiratory circuit which includes the respirator tubings, connectors, passageway, humidifier and accessories which may be associated with them. In those patients with stiff lungs and small lung capacities (e.g., infants with haline membrane disease), the inspiratory effort required to trigger the respirator may become prohibitive, especially if the evacuation volume in the inspiratory circuit is relatively large compared to the capacity of their lungs. Thus, inspiratory triggering by such patients is often difficult, if at all possible.
Prior attempts in overcoming these problems have included various means of detecting chest wall movement or air flow near the airway opening using plethysmographic methods or flow transducers, respectively. Thus, lung expansion, instead of airway pressure, is used as the triggering signal. These approaches require relatively elaborate instrumentation that is cumbersome, costly, and difficult to operate. Bulky and delicate volume or flow sensors must be attached directly to the patient. None of these methods have been proven effective in allowing sensitive and reliable inspiratory triggering.
Ideally, an inspiratory triggering device should be sensitive enough to provide the desired triggering with minimum encumbrance to the patient, and yet simple enough to be relatively light-weight, flexible, and low-cost. Furthermore, it should not adversely affect the well being of the patient or cause unnecessary inconvenience to the therapist in its operation. Such a device is presently lacking.
On the other hand, an advantage of pressure triggering is that a pressure signal can be readily obtained by way of a side tap to the respiratory circuit proximal to the patient while direct attachment of the pressure transducer to the patient is not necessary. Its disadvantage, however, as used in current practice, is that the airway pressure that provides the trigger can be greatly attenuated in the presence of a large evacuation volume in the respiratory circuit. Apparatus that eliminates or minimizes the effect of the evaluation volume would be useful in providing sensitive pressure triggering and, hence, an object of the present invention.